Ask Michael, KB9VBR: What is Reactance and Resonance

[@RSwrightMD]

Joe is a natural teacher! Wow what a simple explanation for calculating impedance.

[@Ne9n]

easy way to think of it. Reactance is the opposition to an Alternating current. Resistance is the opposition to all current. Inductive and capacitance reactance oppose each other, and by changing the inductance and capacitance you can get them to cancel out ( low net reactance) . When Capacitance and inductive reactance have equal values (cancel), resonance is achieved. So there is no opposition to the AC signal and more of the energy may be radiated. It might be good sometime to talk about what impedance matching really means.

[@EricFullwood]

Well done

[@KB9VBRAntennas]

Great explanation. When reactance is cancelled out, any mismatch on the antenna system will be purely resistive. This will be manifest in elevated SWR as a 50 ohm impedance match is not being presented to the transceiver. I think that's a concept Joe and I will have to tackle in a future video.

[@Ne9n]

50ohm for coax was selected as a compromise in between best power handling (30 ohms ) and lowest loss (75 ohms) for air dielectric coax that was used in the 1940's. it drove a standard. Polyethylene filled (delelectic) coax has is 50ohms impedance for the ratios of diameters are (shield to center conductor) are about 3.6. so 50 ohms is a good choice.

[@dandypoint]

Tough subject to tackle. At resonance we only have resistance to worry about and working with and transforming resistance is easy. Throw reactance in to the equation and it is not simple. I have seen amplifiers that will work into a 2:1 SWR, however the actual impedance can be 25 ohms or 100 ohms with no reactance or it can be some resistance with inductive reactance or some resistance with capacitive reactance . Almost an unlimited set of possibilities and still have 2:1 SWR. However the tuning of the amp changes drastically depending on the R and X. Sometimes the amp is not capable of matching some of these loads due to the combination of resistance and reactance. If radiation resistance is 50 and coax is 50 we have a perfect match if and only if the reactance is Zero. However if the resistance is 50 and we have reactance we will not have a 1:1 SWR and we must tune out the reactance with opposite kind to both achieve resonance and obtain that 1:1 SWR. If we have 100 ohms and no reactance we can transform the 100 to 50 easily with a transformer. If there is reactance it’s not quite so easy.

[@johnbauman4005]

Thanks!

[@KB9VBRAntennas]

Thank you for your support, it is greatly appreciated.

[@johnarigot1701]

great video thanks

[@BlackHamRadioUniversity]

What is the purpose of a 2KV capacitor on the inside of an antenna? What is its purpose?

[@TimothyJesionowski]

Capacitors are usually measured in Farads. Do you mean a resistor?

[@BurtonChristmasLites]

Would the same principal be applied for a 9:1 unun in a sloper config?

[@daveN2MXX]

Thank you for trying to explain reactance and resonance. It is my understanding that while it is true that net reactance is close to zero when an antenna (or system) is resonant, zero net reactance does NOT mean that the antenna system is resonant. It could, but not necessarily. Is that statement incorrect? Also, an antenna doesn't have to be resonant to radiate efficiently.....case in point, a 5/8th wave mobile vertical 2m whip.

[@KB9VBRAntennas]

You are correct, and I can think of several non resonant antennas that exhibit close to zero reactance. When we simplify concepts, we achieve a greater level of understanding, only to have edge cases mess up the whole system.

[@brianfields4479]

Very interesting indeed, not fully understood to be honest, and I have no idea what my ants reactance or impedance etc is, but I know it works extremely well, with qso's to the other side of the world everyday. If I see my swr is not through the roof and iam working long haul dx iam happy. Not sure what's happening with my 40m vertical on 30m, swr is really high, but with a tuner it works fantastic when it really shouldn't I don't think. I would hesitate to say, regardless of many things, if it works for you, just leave it hi.

[@arthurgumbus3969]

Is Return Loss in dB important for dialing in an antenna? It seems to coincide with SWR to some extent. So what I sense is that there is low SWR for the Wrong Reasons AND low SWR for the Right Reasons... I think that topic is one that you both should further explore.... with examples.... Art W1SWL

[@KB9VBRAntennas]

Return loss is just another way to quantify SWR. The lower your SWR, the higher your return loss value (in -dB) will be. But to your point, there are right and wrong for an SWR value to be present. If your antenna is resonant any SWR will be purely resistive, that is, not a pure 50 ohm match. If the resistance is within the tolerances of the transmitter, you can operate without using a transformer to get a perfect match (usually 2:1 or less). And that's the point I was trying to make with the Sporty Forty. I could have used to my tuner to 'fix' the match, but opted not to add another level of inefficiency into my antenna system. As you can see, we can get deep into the weeds really quick when discussing resonance, resistance, and reactance. Entire books have been written on the subject with more math than you can shake a stick at. Maybe Joe and I can break this stuff down more in future videos.

[@user-wd1tp7gt9r]

The sound was really bad and with graph or any illustration it will be more informative

[@johnbauman4005]

Have watched this a couple times and have passed the Extra exam, but still not on the air... 07:11: In this example then, would you want to insert a capacitor to offset the excess 10 ohm inductive reactance and bring net reactance to zero? If so, is there a formula to estimate the size capacitor required? Or what is an acceptably low reactance? Earlier you seem to dismiss the concept of trying to reduce SWR to as near 1:1 as possible. This is intriguing because everyone else seems to state the opposite and vendors have inserted limited SWR matching circuits in some radios. Can you clarify? Sorry to be dense but I'm clearly missing a concept or three. Really appreciate your channel! 73, John KK7JBZ

[@SMShannon55]

You’re not being dense. You asked some good questions. There are formulas that calculate the impedance of a capacitor or an inductor at a particular frequency. So, for any specific frequency there’s an inductance and capacitance that cancel out each other. For an inductor, the impedance in Ohms equals 2*Pi*frequency*L where the frequency is in Hertz and the inductance, L is in Henries. For a capacitor, the impedance is the inverse of that: impedance in Ohms = 1/(2*Pi*frequency*C) where again frequency is in Hertz and C is the capacitance measured in Farads. Because we use frequencies in the millions (sometimes billions) we end up with inductance and capacitance that are more easily expressed using prefixes such as milli, micro, and pico.